Technical Abstract:
Peroxidase/H2O2-mediated radical coupling of hydroxycinnamaldehydes produced 8-O-4-, 8-5-, 8-8-, and 5-5-dimers as had been documented earlier (although we found that the 8-5-dimer is produced in its cyclic phenylcoumaran form at neutral pH). Spectral data from dimers and oligomers has allowed a more substantive assignment of aldehyde components in lignins sisolated from a CAD-deficient pine mutant and an antisense-CAD- downregulated transgenic tobacco. The CAD-deficient pine lignin shows the typical benzaldehyde and cinnamaldehyde peaks at levels significantly enhanced over the corresponding lignin from normal pine, along with evidence for two types of 8-O-4-coupled coniferaldehyde units. The CAD- downregulated tobacco has even higher levels of hydroxycinnamaldehyde (mainly sinapaldehyde) incorporation producing significant levels of the analogous two types of 8-O-4-coupled products. 8-8-Coupled units are also clearly evident. The isolated lignins do not appear to be artifacts; they are high molecular weight (~17 kDa), and there is clear evidence for coupling of hydroxycinnamaldehydes with each other and then incorporating into the lignin, as well as for the incorporation of coniferaldehyde monomers onto pre-formed lignin oligomers. The implication is that coniferaldehyde and sinapaldehyde (as well as vanillin and syringaldehyde) co-polymerize with the traditional monolignols, the hydroxycinnamyl alcohols (sinapyl, coniferyl, and 4-hydroxycinnamyl alcohol), into lignins and do so at markedly enhanced levels when the normal monolignol production is downregulated by CAD-deficiency.